It would be invalid due to a defect in form.
Can. 1108 §1. Only those marriages are valid which are contracted before the local ordinary, pastor, or a priest or deacon delegated by either of them, who assist, and before two witnesses according to the rules expressed in the following canons and without prejudice to the exceptions mentioned in cann. ⇒ 144, ⇒ 1112, §1, ⇒ 1116, and ⇒ 1127, §§1-2.
Bottom line is that it would have to be convalidated, in the event that she wanted to come back to the Church. This, of course, assumes that Sarah did not receive a dispensation from her local pastor or ordinary, per Canon 1118.
I missed the last canon, but yeah, you are correct I think.